The present invention relates to a device for applying a liquid, such as a photographic photosensitive liquid, a magnetic liquid, or a surface protective liquid to a flexible carrier (also referred to herein as a web) such as a plastic film, paper, or foil.
Conventional application devices for applying a liquid to a web are well known and generally are, for example, of the roller type, bead type, slide coating type, or extrusion type. The width of each of the devices is wider than the width of the web. The width of the web can be divided into three portions. That is, the width of the web includes one central portion having a predetermined width and two edge portions having a predetermined width approximately in the range of a few to several millimeters. The liquid is applied to the central portion (i.e., liquid-applied portion) but is not applied to the edge portions (i.e., liquid-free portions).
In the extrusion-type device where the liquid is applied to the web while the application head is pushed toward the moving web, as described in the Unexamined Japanese Patent Applications Nos. 84771/82 and 94657/84, the head comes into contact with both side edge portions of the web because the gap between the head and the web is very small. As a result, the application head scrapes the surfaces of the side edge portions of the web causing scraped chips to cling to the end parts of the application edge portion of the head. In addition, foreign matter tends to cling to the surface of the web and become caught on the end parts of the application edge portion of the head. Because the scraped chips and the foreign matter accumulate on the end parts of the application edge portion of the head, the web is pushed up at both side edge portions, and therefore the thickness of the liquid applied to the web is not uniform along the width of the web. In other words, the liquid is not evenly applied, and the thickness of the applied liquid is greater near the side edge portions of the web than the central portion. This is a problem.
To solve this problem, the following application method and device have been proposed. In the proposed method, a solvent is applied to both the liquid-free side edge portions of the web immediately before a liquid is applied to the liquid-applied central portion of the web. Such a method is disclosed in Japanese Unexamined Patent Application No. 257268/86. In the proposed device, the surfaces of both end parts of the application edge portion of the application head, which correspond to the liquid-free side edge portions of the web, slope downward from the surface of the intermediate part of the application edge portion, or they are made smaller in height than the surface of the intermediate part. Thus, a gap is defined between the surface of the liquid-free side edge portions of the web and the surface of the end part of the application edge portion of the head. Such a structure is disclosed in Japanese Patent Application No. 52069/90. However, in this method, the solvent is likely to enter the surface of the liquid-applied portion of the web and/or flow around to the opposite surface of the web due to disturbance such as fluctuations or meandering of the web while it moves along the path rollers, making the thickness of the applied liquid on the web nonuniform. This too is a problem. The width of the solvent applied to each liquid-free side edge portion of the web is increased by the application edge portions (i.e., the back edge portion and doctor edge portion) of the application head. It is difficult to adequately control the width of the applied solvent on the liquid-free side edge portion of the web if the width Of the liquid-free portion is small or the amount of scraping of the doctor edge portion of the application head is high. This also is a problem.
If the solvent enters the surface of the liquid-applied portion of the web or flows around to the opposite surface of the web, for instance during the manufacturing of a magnetic recording medium, a subsequently applied magnetic liquid will unavoidably be caused to cling to a conveyance roller driving the nonmagnetic carrier, as a result of which the liquid contaminates the carrier and a calendering roller used for processing the surface of the carrier after the application of the liquid to the carrier. This causes a problem in that the surface of the carrier is flawed or scratched due to the foreign matter and the like, thereby deteriorating the magnetic recording properties of the medium. Moreover, if the web is a flexible carrier having a small thickness of approximately 15 to 40 .mu.m, the rigidity of the web is so low that the web sags greatly at both side edges toward the application edge portions of the application head. This results in a problem that the side edges of the web come into contact with the edges of the sloped or height-reduced surfaces of the end parts of the application edge portions of the head so as to be cut or scraped.
Recently, the web of a magnetic recording medium for video use has been made of a polyethylene terephthalate base to which spherical grains Of SiO.sub.2 are added as a filler to reduce the amount of friction between the medium and the contact portion of the guide posts of a magnetic recording/playback machine to thus stabilize the running of the medium in the machine. However, the rigidity of the polyethylene terephthalate base containing the filler is so low that the web is likely to sag greatly at both side edges thereof toward the application edge portion of the application head when a liquid is applied to the web. Furthermore, when the web comes into contact with the application edge portion of the head, the filler is likely to come off the web so that a large quantity of scraped dust accumulates on the application edge portion. This too is a problem.
To solve the foregoing problems of the above-described methods and devices, another application device has been proposed in Japanese Patent Application No. 320546/88. In this device, both end parts of the application edge portion of the application head, which correspond to the liquid-free side edge portions of a web, are provided with notches extending in the direction of the movement of the web, and shoulders of the end parts of the application edge portion, which define the notches, provide support for the web at the side edges thereof. Because of the structure of this application device, even if the thickness of the web is small, the liquid-free portions of the web are not scraped by the application edge portions of the application head near the liquid-applied portion of the web and no foreign matter clinging to the surface of the web is caught on the head at the ends thereof.
However, in an application device in which the end parts of the application edge portion of the application head have notches and shoulders as just described, the liquid-free side edge portions of the web need support to prevent them from rubbing against the end parts of the application edge portion of the head, at least near the liquid-applied portion of the web. As a result, the width of each of the notches and the height of each of the shoulders are determined in terms of the thickness of the web. Therefore, it is necessary to adjust the application head every time the thickness or quality of the web is changed. This causes another problem in that the efficiency of production using the device is reduced.